Results from observational studies suggest a small beneficial association between protein and blood pressure that may be mainly attributable to the intake of protein from plant sources. In epidemiological studies biomarkers of dietary intake are useful to estimate intake more reliably. Dietary studies using biomarkers of protein intake generally show stronger associations with health outcomes. However, data on biomarkers for specific types of protein (e.g. from dairy, meat, grain and legumes) are scarce. Therefore the purpose of this study is to identify new biomarkers for the intake of protein from meat, dairy and grain. In addition the secondary purpose is to investigate whether intake of protein from these sources influences kidney filtration rate and urinary acid excretion differentially.